1. Field of the Invention
This invention relates to satellite broadcast methods, and more particularly to multi-beam satellite transmission in which different beams use the same co-polarized frequency spectrum and can interfere with each other.
2. Description of the Related Art
A satellite broadcast system has been developed that is particularly useful for direct television broadcast to relatively small local service areas, as well as to larger regional areas. The system, described in co-pending U.S. patent application Ser. No. 09/160,681 filed on the same day as this application and entitled "Non-uniform Multi-beam Satellite Communications System and Method", employs a series of relatively small "spot" beams to provide local service to higher population markets. A frequency re-use scheme is used in which at least some of the separate beams are broadcast re-using the same frequency spectrum in a non-uniform beam pattern. The target areas for beams which use the same frequencies must not overlap, thereby avoiding excessive interference. Priories are established among different target areas by assigning different sizes and powers to their respective transmission beams, with the higher power beams accommodating a larger number of station signals and also resulting in a lower interference level from other beams. The priorities among different target areas can also be set by the selection of antenna reflector sizes to produce different roll-off characteristics for different beams, and by varying the illumination tapers of different antenna feedhorns to establish different peak-to-edge power differentials for different beams.
To optimize the system's frequency re-use efficiency, some amount of interference is permitted between nearby beams using overlapping or equal frequency bands. For satellite signal transmissions that,are performed digitally, cross-beam interference and thermal noise are generally not perceived as a constant degradation in the signal quality when the system is operating according to design. Rather, higher noise and interference levels can increase the duration and frequency of total signal outages during rain, thunderstorms or other bad weather conditions. The problem for a properly operating system is not one of signal quality, which is always high for a digital system when the signal is received, but of the number and duration of outages. In the past this has been addressed by spacing beams with different signals in the same frequency band so far apart that there is essentially no overlap between them, even in their peripheral areas.
With the system described in co-pending Ser. No. 09/160,681, some degree of cross-beam interference is permitted but it is kept to tolerable levels within each target area (relative to the importance of the market served in that area). However, if the satellite's worse case performance for a given target area is slightly worse than expected, the transmission may not work at all in the peripheral portions of the target area, where the ratio of the beam's carrier power C to the cross-beam interference I is dropping rapidly. The C/I ratio increases towards the center of the beam, where the transmission is limited only by weather conditions which can produce temporary outages, as opposed to the more remote areas where there can be no successful transmission at all if interference is worse than expected.
Even if a satellite broadcast system conforms exactly to the original design, circumstances can change in a way that would obsolete the originally designed interference levels. For example, a higher outage rate will generally be tolerated for an area that has both a low overall population and a low population density. A greater loss of service to this type of area would be a reasonable trade-off if it resulted in a commensurate enhancement of service to an adjacent area with a much higher population. However, if the sparsely populated area is growing rapidly, over time its population may increase to a level at which a relatively high outage rate is no longer acceptable. In this situation the original design will be self-defeating, since it will ultimately result in the assignment of outage rates to different areas on a basis other than the relative importance of the current markets in those areas. Furthermore, as the population within a particular market area increases there is a tendency to enlarge the number of different station signals within its channel. This reduces the power available for each individual station signal and leads to an absolute increase in interference effects from adjacent beams, which is exactly the opposite of what is normally desirable for an area with a rapidly growing population.